The rRNA gene from the protozoan Tetrahymena pyriformis is being used as a model system for studying transcription of the rRNA precursor and its processing both in vivo and in vitro. The rRNA precursor in vivo, and labeled with P32O4, has been isolated and attempts are being made to isolate the 5' and 3' terminal oligonucleotides. The oligonucleotides will be employed as an assay for specific initiation and termination of transcription in vitro. The in vivo labeled rRNA precursor is being used as a substrate for identifying and isolating processing nucleases. The in vivo synthesized p25S rRNA and p17s rRNA processing intermediates are being isolated and the 5' and 3' terminal oligonucleotides will be sequenced. The terminal oligonucleotides of the in vitro processing products will be determined and compared to those found in vivo in order to determine the specificity of the processing endonucleases in vitro.